Drop redundant index on ci_pipelines.project_id

According to
https://gitlab.com/gitlab-com/gl-infra/infrastructure/issues/8823, there
are numerous redundant indexes for index_ci_pipelines_on_project_id:

* index_ci_pipelines_on_project_id_and_id_desc
* index_ci_pipelines_on_project_id_and_ref_and_status_and_id
* index_ci_pipelines_on_project_id_and_sha
* index_ci_pipelines_on_project_id_and_source
* index_ci_pipelines_on_project_id_and_status_and_config_source
* index_ci_pipelines_on_project_id_and_status_and_updated_at
* index_ci_pipelines_on_project_idandrefandiddesc

This index is taking 3+ GB, so let's drop it.

changelogs/unreleased/sh-drop-ci-pipelines-redundant-index.yml

+---
+title: Drop redundant index on ci_pipelines.project_id
+merge_request: 22325
+author:
+type: other

db/migrate/20191229140154_drop_index_ci_pipelines_on_project_id.rb

+# frozen_string_literal: true
+
+class DropIndexCiPipelinesOnProjectId < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index :ci_pipelines, :project_id
+  end
+
+  def down
+    add_concurrent_index :ci_pipelines, :project_id
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_12_18_225624) do
+ActiveRecord::Schema.define(version: 2019_12_29_140154) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -871,7 +871,6 @@ ActiveRecord::Schema.define(version: 2019_12_18_225624) do
     t.index ["project_id", "source"], name: "index_ci_pipelines_on_project_id_and_source"
     t.index ["project_id", "status", "config_source"], name: "index_ci_pipelines_on_project_id_and_status_and_config_source"
     t.index ["project_id", "status", "updated_at"], name: "index_ci_pipelines_on_project_id_and_status_and_updated_at"
-    t.index ["project_id"], name: "index_ci_pipelines_on_project_id"
     t.index ["status"], name: "index_ci_pipelines_on_status"
     t.index ["user_id"], name: "index_ci_pipelines_on_user_id"
   end